Stofen Omni Bounce flash diffusers have quickly gained a reputation for ease of use, compactness and effective flash control in many different conditions. They're designed to be used by bouncing light off of the ceiling, but can also be used as an intermediate surface with direct flash. They simply snap onto the front of the flash and are easily removed.
Omni Bounces decrease the effective range of the flash, but will not otherwise affect the performance of the TTL system - making them simple to use as long as you are aware of the decreased range. All flash diffusers decrease the effective guide number, but many do this in addition to requiring flash exposure compensation. Omni Bounces are able to expand the range of coverage of your flash, permitting shorter focal length lenses to be evenly illuminated by artificial light.
The Green Omni Bounce is used to correct for Fluorescent Lighting in conjunction with an FLD or 30M filter on the camera lens.